Wavelength modulated fiber optic sensors, in particular fiber Bragg grating (FBG) sensors, have been applied to many sensing applications. See, for example, A. Othonos, “Bragg Gratings in Optical Fibers: Fundamentals and Applications”, in Optical Fiber Sensor Technology, K. T. V. Grattan and B. T. Meggitt, eds. pp.79-188, Kluwer Academic Publishers, Boston, 2000. The most important advantage of this type of sensor is that wavelength is an absolute parameter and not affected by the losses in the system or fluctuations in the source power.
For field applications, the wavelength interrogator (which is a key component of the sensor system) is required to have the characteristics of portability, ruggedness, low cost, high measurement accuracy, high speed and multiplexing capability. However, none of the traditional methods is enough satisfactory for those requirements. In recent years, arrayed waveguide gratings (AWO) based interrogation systems have shown great potential for fulfilling all those requirements. One technique described by Y. Sano and T. Yoshino, entitled “Fast optical wavelength interrogator employing arrayed waveguide grating for distributed fiber Bragg grating sensors”, J. Lightwave Techno. Vol. 21, pp. 132-139, 2003, involves taking the ratio of the intensities in adjacent AWG channels when the fiber Bragg grating (FBG) wavelength lies between the two channels. This simple approach yielded good performance but suffers from a limited usable range (less than the channel spacing) and a reduced sensitivity near the extremes of the range.
D. C. C. Norman, D. J. Webb and R. D. Pechstedt, “Extended range interrogation of wavelength division multiplexed fibre Bragg grating sensors using arrayed waveguide grating”, Electro. Lett. Vol. 39, pp. 1714-1715, 2003 overcame those drawbacks by using a heterodyne approach based on interferometric wavelength shift detection. Nevertheless, it makes the interrogation system much more complicated.
We have proposed another interrogation approach using an AWG based demultiplexer. This approach is based on the idea that by changing the temperature of an AWG, the transmission wavelength of one of its channels can be tuned to the sensor wavelength. Thus we are able to correlate the sensor wavelength to the AWG temperature.